In an optical system for projection a reduced image of a photomask on a substrate in UV micromachining operations it is desirable to provide a high intensity of illuminating radiation at the photomask and to transfer as much as possible of the illumination of the photomask into the reduced image of the photomask on the substrate. A commonly used projection optic for projecting the mask image is a Schwarzschild objective. A Schwarzschild objective is one, well-known, embodiment of an on-axis (the optical axis of the projection system) reflective telescope, used in reverse, in which light from the illuminated photomask passes through a primary mirror having a concave reflective surface via a central aperture in the primary mirror, and is reflected from an on-axis convex secondary mirror onto the concave reflective surface of the primary mirror. Here, it should be noted that the terms “primary mirror” and “secondary mirror”, used here and in the description of the invention presented herein below are the terms that are normally used in designating mirrors of a reflective telescope and do not reflect the order in which radiation strikes the mirrors when a reflective telescope is used in reverse as a reducing objective.
An all-reflective objective such as the Schwarzschild objective has an advantage that reflective optical elements can provide less loss of (UV) light through absorption of the light than refractive elements. The Schwarzschild and similar all reflective objectives have a partially offsetting disadvantage that light entering the objective close to the axis or at low inclination to the axis is reflected from the secondary mirror, back through the aperture in the primary mirror, creating, in effect, a “dark zone” or zone of obscuration on the secondary mirror close to the axis.
In prior-art optical systems, designs to minimize the effect of the dark zone in causing energy transfer losses have employed using a costly, difficult-to-implement system of cylindrical lenses and beam homogenizing elements to evenly distribute illumination light across the photomask and thereby reduce concentration of illumination on the central dark zone of the secondary mirror. Such a system is described in detail in German Patent Publication DE 195 33 314, the complete disclosure of which is hereby incorporated by reference. There is a need for a UV optical projection system that minimizes transfer losses due to the dark zone of a Schwarzschild objective or the like, but that is less costly and less difficult to implement than optical systems of the type described in the '314 publication.